In general, noise that is present in a listening environment nearly always compromises the experience of listening to audio through headphones. For instance, in an airplane cabin, noise from the airplane produces unwanted acoustic waves, i.e., noise, that travel to the listener's ears, in addition to the audio program. Other examples include computer and air-conditioning noise of an office or house, vehicle and passenger noise in public or private transportation, or other noisy environments.
In an effort to reduce the amount of noise received by the listener, two major styles of noise reduction have been developed, passive noise reduction and active noise cancellation. Passive noise reduction refers to a reduction in noise caused by placing a physical barrier, which are commonly headphones or earplugs, between the ear cavity and the noisy outside environment. The amount of noise reduced depends on the quality of the barrier. In general, noise-reduction headphones having more mass provide higher passive noise reduction. Large, heavy headphones may be uncomfortable to wear for extended periods, however. For a given headphone, passive noise reduction works better to reduce the higher frequency noise, while low frequencies may still pass through a passive noise reduction system.
Active noise reduction systems, also called active noise cancellation (ANC), refers to the reduction of noise achieved by playing an anti-noise signal through headphone speakers. The anti-noise signal is generated as an approximation of the negative of the noise signal that would be in the ear cavity in absence of ANC. The noise signal is then neutralized when combined with the anti-noise signal.
In a general noise cancellation process, one or more sensors (e.g. microphones) monitor ambient noise or noise in the earcups of headphones in real-time, then the system generates the anti-noise signal from the ambient or residual noise. The anti-noise signal may be generated differently depending on factors such as physical shape and size of the ANC system, (e.g., headphones, etc.), frequency response of the sensor and a transducer, e.g. speaker, latency of the transducer at various frequencies, sensitivity of the sensor, and placement of the transducers and sensors, for example. The variations in the above factors between different sensors and transducers (e.g., headphones) and even between the two ear cups of the same headphone system mean that optimal filter design for generating anti-noise also vary.
Latency in processing an anti-noise signal prevents Active Noise Cancellation systems from operating efficiently. For instance, digitizing the sensor signals and processing the signal at rates common in audio processing, such as 44.1 KHz or 48 KHz introduces large latency. Because performance of an acoustic processor, such as an ANC, depends on the ability to detect noise and produce the anti-noise signal soon enough in time to cancel the noise, a large latency is detrimental to acoustic noise cancellation processing.
Embodiments of the invention address this and other limitations of the prior art.